The role of the right ventricle during hypobaric hypoxic exercise: insights from patients after the Fontan operation

Objectives: The principal objective of this study was to examine the import ance of the right ventricle for maximal systemic oxygen transport during ex ercise at high altitude by studying patients after the Fontan operation. Ba ckground: High-altitude-induced hypoxia causes a reduction in maximal oxyge n uptake. Normal right ventricular pump function may be critical to sustain cardiac output in the face of hypoxic pulmonary vasoconstriction. We hypot hesized that patients after the Fontan operation, who lack a functional sub pulmonary ventricle, would have a limited exercise capacity at altitude, wi th an inability to increase cardiac output. Methods: We measured oxygen upt ake (VO2, Douglas bag), cardiac output (Qc, C2H2 rebreathing), heart rate ( HR) (ECG), blood pressure (BP) (cuff), and O-2 Sat (pulse oximetry) in 11 p atients aged 14.5 +/- 5.2 yr (mean +/- SD) at 4.7 +/- 1.6 yr after surgery. Data were obtained at rest, at three submaximal steady state workrates, an d at peak exercise on a cycle ergometer. All tests were performed at sea le vel (SL) and at simulated altitude (ALT) of 3048 m (10,000 ft, 522 torr) in a hypobaric chamber. Results: At SL, resting O-2 sat was 92.6 +/- 4%. At A LT, O-2 sat decreased to 88.2 +/- 4.6% (P < 0.05) at rest and decreased fur ther to 80 +/- 6.3% (P < 0.05) with peak exercise. At SL, VO2 increased fro m 5.1 +/- 0.9 mL.kg(-1).min(-1) at rest to 23.5 +/- 5.3 mL.kg(-1) .min(-1) at peak exercise and CI (Qc.m(-2)) increased from 3.3 +/- 0.7 L.m(-2) to 6. 2 +/- 1.2 L.m(-2).VO2 peak, 17.8 +/- 4 mL.kg(-1).min(-1) (P < 0.05), and CI peak, 5.0 +/- 1.5 L.m(-2) (P < 0.05), were both decreased at ALT. Remarkab ly, the relationship between Qc and VO2 was normal during submaximal exerci se at both SL and ALT. However at ALT, stroke volume index (SVI, SV.m(-2)) decreased from 37.7 +/- 8.6 mL.min(-1).m(2) at rest, to 31.3 +/- 8.6 mL.min (-1).m(2) at peak exercise (P < 0.05), whereas it did not fall during sea l evel exercise. Conclusions: During submaximal exercise at altitude, right v entricular contractile function is not necessary to increase cardiac output appropriately for oxygen uptake. However, normal right ventricular pump fu nction may be necessary to achieve maximal cardiac output during exercise w ith acute high altitude exposure.